private void recheck() {
    // If this is the oldest node.
    if (oldestNode.get().id().equals(cctx.localNodeId())) {
      Collection<UUID> remaining = remaining();

      if (!remaining.isEmpty()) {
        try {
          cctx.io()
              .safeSend(
                  cctx.discovery().nodes(remaining),
                  new GridDhtPartitionsSingleRequest(exchId),
                  SYSTEM_POOL,
                  null);
        } catch (IgniteCheckedException e) {
          U.error(
              log,
              "Failed to request partitions from nodes [exchangeId="
                  + exchId
                  + ", nodes="
                  + remaining
                  + ']',
              e);
        }
      }
      // Resend full partition map because last attempt failed.
      else {
        if (spreadPartitions()) onDone(exchId.topologyVersion());
      }
    } else sendPartitions();

    // Schedule another send.
    scheduleRecheck();
  }
  /** @throws Exception Thrown if test failed. */
  public void testC() throws Exception {
    final Collection<SampleBean> set = new GridConcurrentWeakHashSet<>();

    int threadCnt = 2;

    final int cnt = 5;

    final CountDownLatch start = new CountDownLatch(1);
    final CountDownLatch stop = new CountDownLatch(threadCnt);

    Runnable r =
        new Runnable() {
          @Override
          public void run() {
            try {
              start.await();

              for (int i = 0; i < cnt; i++) {
                for (int j = 0; j < cnt; j++) set.add(new SampleBean(i));
              }
            } catch (Exception e) {
              error(e.getMessage());
            }

            stop.countDown();
          }
        };

    for (int i = 0; i < threadCnt; i++) new Thread(r).start();

    start.countDown();

    stop.await();

    assert set.size() == cnt;

    gc();

    assert set.isEmpty();
  }
  /** @throws Exception Thrown if test failed. */
  public void testA() throws Exception {
    Collection<Integer> set = new GridConcurrentWeakHashSet<>();

    Integer i = 1;

    assert set.add(i);
    assert !set.add(i);

    assert set.contains(i);

    assert set.size() == 1;

    Collection<Integer> c = F.asList(2, 3, 4, 5);

    assert set.addAll(c);
    assert !set.addAll(c);

    assert set.containsAll(c);

    assert set.size() == 1 + c.size();

    assert set.remove(i);
    assert !set.remove(i);

    assert !set.contains(i);

    assert set.size() == c.size();

    assert set.removeAll(c);
    assert !set.removeAll(c);

    assert !set.containsAll(c);

    assert set.isEmpty();

    Collection<Integer> c1 = Arrays.asList(1, 3, 5, 7, 9);

    int cnt = 0;

    for (Iterator<Integer> iter = set.iterator(); iter.hasNext(); cnt++) c1.contains(iter.next());

    assert set.size() == cnt;

    assert set.size() == set.toArray().length;

    assert set.addAll(c1);

    assert set.retainAll(c);
    assert !set.retainAll(c);

    Collection<Integer> c2 = F.retain(c1, true, c);

    assert set.containsAll(c2);
    assert !set.containsAll(c1);
    assert !set.containsAll(c);

    assert set.size() == c2.size();

    set.clear();

    assert set.isEmpty();

    try {
      set.iterator().next();

      assert false;
    } catch (NoSuchElementException ignored) {
      assert true;
    }

    try {
      set.add(null);

      assert false;
    } catch (NullPointerException ignored) {
      assert true;
    }
  }
  /** @throws Exception Thrown if test failed. */
  public void testD() throws Exception {
    final Collection<SampleBean> set = new GridConcurrentWeakHashSet<>();

    final int cnt = 100;

    final CountDownLatch start = new CountDownLatch(1);
    final CountDownLatch stop = new CountDownLatch(3);

    new Thread() {
      @Override
      public void run() {
        try {
          start.await();

          for (int i = 0; i < cnt; i++) {
            for (int j = 0; j < cnt; j++) set.add(new SampleBean(i));
          }
        } catch (Exception e) {
          error(e.getMessage());
        }

        stop.countDown();
      }
    }.start();

    new Thread() {
      @Override
      public void run() {
        try {
          start.await();

          for (int i = 0; i < cnt; i++) {
            for (int j = 0; j < cnt; j++) set.remove(new SampleBean(i));
          }
        } catch (Exception e) {
          error(e.getMessage());
        }

        stop.countDown();
      }
    }.start();

    new Thread() {
      @SuppressWarnings({"UnusedDeclaration"})
      @Override
      public void run() {
        try {
          start.await();

          while (stop.getCount() > 1) {
            for (SampleBean b : set) {
              // No-op.
            }
          }
        } catch (Exception e) {
          error(e.getMessage());
        }

        stop.countDown();
      }
    }.start();

    start.countDown();

    stop.await();

    gc();

    assert set.isEmpty();
  }
  /** @throws Exception Thrown if test failed. */
  @SuppressWarnings({"UnusedAssignment"})
  public void testB() throws Exception {
    Collection<SampleBean> set = new GridConcurrentWeakHashSet<>();

    SampleBean bean1 = new SampleBean(1);

    assert set.add(bean1);
    assert !set.add(bean1);

    assert set.size() == 1;

    assert set.contains(bean1);

    bean1 = null;

    gc();

    assert set.isEmpty();

    Collection<SampleBean> c =
        F.asList(new SampleBean(1), new SampleBean(2), new SampleBean(3), new SampleBean(4));

    assert set.addAll(c);
    assert !set.addAll(c);

    assert set.size() == c.size();

    assert set.containsAll(c);

    c = null;

    gc();

    assert set.isEmpty();

    SampleBean b1 = new SampleBean(1);
    SampleBean b2 = new SampleBean(2);
    SampleBean b3 = new SampleBean(3);
    SampleBean b4 = new SampleBean(4);
    SampleBean b5 = new SampleBean(5);

    set.add(b1);
    set.add(b2);
    set.add(b3);
    set.add(b4);
    set.add(b5);

    Iterator iter = set.iterator();

    assert iter.hasNext();

    b2 = null;
    b3 = null;
    b4 = null;

    gc();

    int cnt = 0;

    while (iter.hasNext()) {
      info(iter.next().toString());

      cnt++;
    }

    assert set.size() == cnt;
  }